Ni–Cr dental alloys - porcelain firing impact on corrosion properties and surface characteristics

Biocompatibility is a critical aspect of the use of materials in the human body. The use of base metal alloys in dentistry is primarily regulated by health and safety standards set by regulatory authorities in various countries. The porcelain-fused-to-metal (PFM) process applied to Ni-Cr dental alloys can alter their properties, particularly in terms of corrosion and surface characteristics. This study aimed to assess the effect of the heat processing used for dental porcelain firing on these properties. The two casted alloys: Ceramic N and Ivoclar Vivadent 4all, used in the study were characterized by analyzing the microstructure by scanning electron microscopy (SEM), composition with energy dispersive x-ray spectroscopy (EDS), hardness, surface profile and electrochemical corrosion resistance (Ecorr, jcorr, polarization curve, Ebr and electrochemical impedance spectroscopy (EIS) results), as well as ions release before and after the simulated porcelain firing. Based on the conducted research the following conclusions can be drawn: Analyzes of the material characteristics before and after the simulation showed that the discussed process, although it does not cause the formation of chemical impurities on the surface of the alloys, results in changes in the chemical composition and structure of surface oxides, increases roughness and reduces hardness. The results of the corrosion examinations showed a deterioration in anti-corrosion properties after the simulation. The statistically significant decrease in corrosion resistance may result from the increased heterogeneity of the surface oxide layers and partial changes in their composition.


Introduction
Owing to the high cost of precious metals, base metal alloys have gained acceptance as a dominant choice for manufacturing removable partial denture frameworks.They have also been adopted for all-metal and metalceramic dentures [1].Base alloy systems include, but are not limited to, cobalt-chromium, nickel-chromium and titanium alloys.Like all biomaterials, base alloys can have harmful effects on the human body.In addition to the desired mechanical properties and aesthetics, metallic dental materials should meet the criteria of biocompatibility, non-toxicity, high corrosion and wear resistance.Titanium and its alloys, including newgeneration intermetallic alloys like TiAl, are indeed among the most sought-after non-precious metal alloys due to their good combination of biocompatibility, corrosion resistance, and mechanical properties [2,3].At the same time, increasingly newer methods are being researched and applied to improve the properties of nonprecious alloys to enhance biocompatibility and reduce the formation of harmful biofilm.Surface modifications include, among others, the use of PVD and CVD coatings [4], bioactive coatings [5], and oxidation/anodizing processes that allow for control over the thickness, and composition of oxide layers and their functionalization with bioactive elements.For example, the surface of cp Ti can be modified by electrolytic plasma oxidation to produce TiO 2 layers containing Ca, P, and Cu with higher TiO 2 density, increased corrosion resistance and improved bactericidal properties due to the addition of Cu [6].Advanced surface treatment techniques also play a key role in counteracting biofilm-related infections on dental implants, such as micro-current wire electrical discharge machining (mWEDM) and laser surface treatment techniques promoting antibacterial activity, thus offering the potential for wide application of Ti alloys, including TiAl, in implants [7].
The use of metals, including dental alloys, is strictly regulated by directives and regulations to protect health.For example, the U.S. Food and Drug Administration (FDA) in Quality System (QS) Regulation/Medical Device Current Good Manufacturing Practices (CGMP) classifies dental base metal alloys in Class II [8].The U.S. Environmental Protection Agency does not have specific regulations regarding nickel levels [9], however, since 2019 FDA's Centre for Devices and Radiological Health (CDRH) has established a Medical Devices Advisory Committee to provide expertise and assistance on the development, safety, and regulation of medical devices [10].In the European Union since May 2021, a new regulation of the use of medical devices (EU 2017/745 -MDR) has been in force [11].
Apart from Ti and Co-Cr alloys, the most commonly used metal alloys for permanent dental restorations have been Ni-Cr alloys.Compared to Co-Cr alloys, they have lower melting ranges, which does not complicate their laboratory processing.The oxide on the surface of Ni-Cr alloys is easier to mask, and the thermal expansion coefficients are often more compatible with porcelain than in the case of Co-Cr alloys [12].However, it is known that Ni can cause respiratory cancer when inhaled in nickel-containing dust and fumes, but not when ingested [13,14].Another serious problem related to the biocompatibility of Ni-Cr alloys is the hypersensitivity to nickel which may exacerbate allergic symptoms due to the release of Ni in the oral cavity environment [15][16][17].
The EU Regulation 2017/745 has impacted the use of materials in dental prosthetics.It imposes stricter regulations on medical devices to increase safety.Specifically, regarding Ni-Cr alloys, their use in dental prosthetics has been analysed due to concerns related to the release of nickel ions.Materials used in dental prosthetics must meet rigorous requirements for corrosion resistance.Manufacturers of dental prosthetics using Ni-Cr alloys have to ensure that their products meet the Medical Devices Registration (MDR) requirements for biocompatibility and are safe for patients, which leads to conducting additional tests and providing evidence of their safety.It should also be noted that the European Union has established two directives: -the release of Ni from parts in direct and prolonged contact with the skin must be less than 0.5 μg cm −2 week −1 [18]; -all metal parts inserted into pierced ears and other parts of the human body must not exhibit a Ni release rate greater than 0.2 μg cm −2 week −1 [19].However, it should be noted that there is a lack of in-depth reference in the literature to the issue of the use of Ni-Cr prosthetic alloys in light of current regulations aimed at protecting health.
Many studies have explored the harmful effects of nickel or Ni-Cr alloys on the human body [20][21][22][23].The literature also includes research comparing the chosen properties of Ni-Cr and Co-Cr alloys in dental applications.The assessment of the corrosion resistance of dental Ni-Cr alloys, especially compared to Co-Cr alloys, is presented ambiguously.The authors report greater corrosion resistance and biocompatibility of the latter compared to Ni-Cr [24][25][26][27], or similar corrosion parameters [28,29] such as the work of Golgovici et al [30], whose authors conducted comparative studies of the morphology, composition and corrosion behavior of Co-Cr and Ni-Cr alloys under the influence of thermal preprocessing.They found that an increase in temperature causes a deterioration of the corrosion resistance of both groups of alloys, showing that Ni-Cr alloys have a lower corrosion resistance compared to Co-Cr alloys.However, although the amounts of ions released increased with increasing temperature, the amounts for Co-Cr and Ni-Cr alloys still did not exceed 20 ppm.
Due to the lower price and easier processing of Co-Cr and Ni-Cr alloys compared to Ti, it is expected that the tendency to eliminate their use, following EU regulations, will be constantly accompanied by research on their corrosion resistance and, of course, on surface modification, new manufacturing technologies and composition changes to reduce toxicological risk [31][32][33][34].
The properties of these two alloy groups are largely influenced by the addition of chromium [35] which quickly oxidizes on the surface to form a thin layer of chromium oxide, prevents oxygen diffusion to the underlying metals and improves their corrosion resistance [36].Also, the addition of molybdenum is of great importance as it increases corrosion resistance and strength and reduces the coefficient of thermal expansion of base metal alloys [36,37].In the fabrication of crowns with a metal framework (PFM system), the latter feature is beneficial for bonding the porcelain veneer, minimizing the risk of porcelain fracture [1].
As mentioned earlier, an inherent problem associated with the use of base metal alloys in prosthetics is their corrosion resistance in contact with physiological fluids [25,[38][39][40].The presence of chloride ions in the environment, very good oxygenation of the electrolyte solution and its relatively high temperature (37 °C) are important in the phenomenon of corrosion of metals and alloys.Constantly changing conditions in the oral cavity (e.g., pH change of saliva, presence of food, microorganisms) are highly unfavorable [41][42][43].
Corrosion affects not only the metal parts of dentures that are in direct contact with saliva but also runs in the gap between the metal substructure and the ceramic face.It mainly concerns the inner surface of metal-ceramic crowns because none of the prosthetic types of cement used for crowns and bridges ensure adequate connection tightness.Thus, the electrolytic environment of the oral cavity penetrates and remains in contact with the surface of the metal (inside of the crown), causing corrosion processes.In special cases, porcelain veneer covers only the vestibular surface of the crown or the bridge.The lingual gingival portion of the crown is often left unlined with porcelain.Areas not covered with porcelain may be a potential cause of ion release due to corrosion processes [44][45][46].
The processes of applying porcelain on a metal framework require the selection of the appropriate type of porcelain.Leucite-reinforced feldspar metal ceramics are dedicated to all non-precious alloys.The quality of ceramics depends on the choice of components and their purity to ensure optical properties and chemical inertness.The appropriate synthesis process of such ceramics also allows for the lowering of the leucite crystallization temperature [47].As is known, the porcelain firing is carried out at high temperatures in the range of 950-1010 °C, which causes irreversible, multidirectional changes in the microstructure of the metal alloy, including homogenization, phase transformation and oxidation.Changes in the microstructure can affect the protective oxide layer, which in turn changes the corrosion resistance of the alloys [48][49][50].Understanding the corrosion behavior of Ni-Cr alloys in the oral environment after dental porcelain firing processes is an important issue in the clinical aspect of their biocompatibility.
The results of studies on the effect of porcelain firing processes on nickel alloys, particularly regarding changes in corrosion resistance, are often conflicting and need additional investigation, especially nowadays when discussions about their toxicological risks are more intense.This also applies to contrasting evaluations of hardness or the quality of oxide layers on the surfaces of these materials before and after heat treatment.Some of the studies report an increase in the corrosion rate of some Ni-Cr alloys after porcelain firing [49][50][51] while others present results showing no negative impact of heat treatment (HT) on corrosion resistance [52,53].Divergent results on the impact of heat treatment on corrosion properties also apply to studies on dental Co-Cr alloys.Some researchers find a higher level of ions released before the HT [54], while others' results indicate no differences in corrosion resistance [55].However, several works reported the reduced corrosion resistance after HT [56][57][58].
In light of the research results to date, it is not groundless to state that it requires further research.
According to the best knowledge of the article's authors, there have been no studies dedicated to comprehensively analysing the effects of porcelain firing processes on Ni-Cr alloy substructures across a wide range of properties of these metallic materials.The primary objective of this work was to determine the corrosion behavior.The manuscript also provides information about the influence of heat treatment on hardness and surface profile, while indicating the correlation of these changes with the detailed alterations in the materials' structure.
We believe that this work can provide a comprehensive picture of the impact of porcelain firing on a range of properties of Ni-Cr alloys, particularly their corrosion resistance and surface condition, and evaluate whether changes in corrosion parameters can be correlated with changes in the composition and structure of oxides on the surface of the investigated alloys.The issue of corrosion resistance in materials used for prosthetics has gained particular importance in light of the current EU regulations, and the test results provided may be important in the context of using these materials following heat treatment related to porcelain firing.

Materials and methods
The study investigates selected properties of elements made with two Ni-Cr alloys, Ceramic N and Ivoclar 4all, before and after the simulated firing process.Chosen Ni-Cr alloys (with proved biocompability, according to the producers' information) are typically intended for metallic crowns, adapted for covering with feldspar ceramics, characterized by moderate hardness and good castability.Differences in chemical composition allowed for the examination and comparison of potential variations in their properties, including corrosion behaviors, in the applied research processes.Table 1 presents the chemical composition of alloys.

Sample preparation
Samples in the form of hollow disks with dimensions of Ø = 14 mm, h = 9 mm were obtained by the traditional casting method using the Pi Dental Silvercast centrifugal induction device (Pi Dental, Hungary) utilizing

The heat treatment simulating the porcelain firing
After degreasing and steam-cleaning specimens of both alloys were subjected to ceramic firing heat treatment.They were heat treated by the time-temperature sequence used in the porcelain firing cycle in a ceramic furnace Heramat C (Heraeus Kulzer, Germany), but without the application of porcelain.Heat treatment was carried out under a vacuum in the temperatures appropriate for the oxidation process and the process of firing individual layers of porcelain onto the metal.The conditions of the oxidation process were averaged for both alloys, and the parameters of the firing process of ceramic layers were set on based on the instructions of Vintage Pro Ceramics by SHOFU (SHOFU Inc., Japan), which is dedicated to all non-precious alloys [59].The whole process was carried out in 8 stages corresponding to the oxidation and firing of individual ceramic layers.Particular processes of heat treatment were as follows: oxidation-for heating in the range from 500 °C to 960 °C with a rate of temperature growth of 100 °C min −1 , then holding in vacuum for 1 min at a temperature of 960 °C.
Ceramics fusion on metal -the heating procedures corresponded exactly to the temperatures of opaquer and dental ceramics fusion.Simulated firing processes are precisely described in table 2, with a division into stages and their approximated duration time.

Sample evaluation
The selection of the research conducted was dictated by the need to scientifically assess how the clinical porcelain firing process affects various properties of Ni-Cr alloys, including their microstructure, surface hardness and profile, as well as the composition of the surface and, consequently, resistance to electrochemical corrosion and release of metal ions.
The samples were subjected to the following types of evaluation: scanning electron microscopy (SEM); energy dispersive x-ray spectroscopy (EDS), profile measurements, hardness measurements and corrosion examinations (E corr , j corr , polarization curve, E br and electrochemical impedance spectroscopy (EIS), as well as ions release measurements).Samples without heat treatment served as a control group.
The following designations were used in the work: Ceramic N, 4all: Ivoclar 4all, Control: sample in an ascast state, before heat treatment; TT: thermally treated sample (after HT simulating porcelain firing).

SEM-EDS study
For the microstructural characterization, all specimens were examined with an SEM using JEOL JSM-6610LV (JEOL, Akashima Tokyo, Japan) with a secondary electron (SE) detector The tested surfaces were imaged under an accelerating voltage of 20 kV.The surface distributions of elements for samples before (control) and after HT were determined by energy-dispersive x-ray spectroscopy at the same accelerating voltage.Microanalysis of the chemical composition was performed using an EDS X-MAX 80 microanalyzer (Oxford Instruments, UK).According to the EDS method, a qualitative and quantitative analysis of the chemical composition of selected areas and points and along selected lines was performed.Three samples from each study and control group were evaluated.

Hardness test
The hardness values of the samples were carried out using the Vickers method with Microhardness Tester FM (Future-Tech, Poland) equipped with the diamond-indenting tool, under a load of 100 g.The results of 5 measurements of both indentation diagonals' average length were converted into Vickers hardness and its average value and standard deviation were determined.

Profilometry
The surface profile was examined using the non-contact method using a Nikon MA 200 metallographic microscope equipped with the C1 confocal system (Nikon MA200 Eclipse, C1 series, Japan).The surface roughness of two Ni-Cr alloys before and after the HT was evaluated using two parameters: Rz-roughness height according to the five highest elevations of the profile and the five lowest depressions on the elementary section, Ra -arithmetic mean deviation of the profile from the mean line (without the information about the shape of the profile).

Corrosion examinations and immersion tests
The electrochemical analyses were performed using a Radiometer-Analytical electrochemical cell (CEC/TH Thermostated Multipurpose Cell -Radiometer Analytical, France).
To compare the corrosion resistance of Ni-Cr samples of both examined alloys before and after the HT, we performed electrochemical tests in a 0.9% NaCl solution (Braun, Melsungen, Germany).The corrosion resistance of the samples was assessed with an ATLAS 0531 Electrochemical Unit & Impedance Analyser kit (Atlas-Sollich, Rebiechowo, Poland)-a five-electrode device for measuring the chrono-volt-amperometric curves of electrochemical systems that is compatible with AtlasCorr software, version 3.19 (Atlas-Sollich, Gdansk, Poland), enabling the control of measurements and the registration of results.
The Ceramic N and Ivoclar 4all samples acted as the test electrodes, while the reference electrode was an Ag/ AgCl electrode in a saturated KCl solution with an electrolyte bridge finished with Lugin's capillary to minimize the electrolyte resistance.The counter electrode was a platinum electrode.All the potentials in this study are given to the Ag/AgCl electrode (E 0 = 0.220 V versus standard hydrogen electrode).The front surface of the cylindrically shaped samples, following grinding and mechanical polishing, was degreased in ethyl alcohol, dried and mounted on a polyamide plastic handle.These assemblies were then placed in a 250 cm 3 electrochemical cell with the other electrodes.The active surface of the test electrode was 0.95 cm 2 .All samples were pre-exposed to the solution for 2 h to establish an open circuit potential (OCP).
In the performed tests, we conducted the following corrosion measurements of the samples: EIS measurements, Tafel characteristics measurement and potentiodynamic polarization tests.Electrochemical Impedance Spectroscopy was performed 2 h after immersion in an aerated solution at open-circuit potential.Impedance spectra were recorded using an AC signal with an amplitude of 10 mV over a frequency range of 10 kHz to 0.01 Hz.Spectra were analyzed in terms of an equivalent circuit (EC) following best-fit EC principles, which involved using the minimum number of circuit elements, with errors of less than 5% for each element.An appropriate equivalent circuit was constructed using the AutoLab software, based on the ZSimpWin program, the obtained data was analyzed using Bode impedance |Z| and Bode phase diagrams.For EIS measurements 5 trials were conducted and the average of the most consistent values was reported.
The Tafel extrapolation method in the domain of the ±150 mV, Ag/AgCl versus the OCP was used to determine the values of corrosion current density j corr and corrosion potential E corr .Five sequences of measurements were taken for all tested variants.The calculations were performed with the AtlasLab software, version 2.9 (Atlas-Sollich, Gdansk, Poland) to determine the corrosion process parameters.Subsequently, cathodic and anodic polarization across the full voltage spectrum (from −0.7V to 1.0 V) using the potentiodynamic method at a potential change rate of 0.167 mV min −1 was executed.The potentials for abrupt current increase (E br ) were estimated from the polarization curves as the potential of the breakdown of the passive film caused by pitting or transpassive dissolution.The value of E br was estimated at the inflection point of the polarization curves.The potential range between E corr and E br represents the passive zone of the alloy.
Immersion tests were performed to evaluate the amount of metal ions released from the Ni-Cr alloys specimens immersed in an acidic artificial saliva solution with pH 2.3 according to ISO 10271.Specimens were incubated in closed sterile centrifuge tubes at 37 °C.At seven days after immersion, the solutions were analysed by inductively coupled plasma-mass spectroscopy (ICP-MS) using matrix-matched standards.The concentrations of Co, Cr, and Mo ions were converted to units of μg/cm 2 (n = 3).

Statistical analysis
All materials characterization data are presented as the average value and standard deviation (SD).Five/three sequences of measurements were taken for all samples.Data obtained from EIS tests, E corr , j corr values obtained through measurements of electrochemical parameters using the Tafel plot method, and E br values derived from potentiodynamic curve analysis, the number of ions released along with hardness measurement results for both alloys, underwent statistical analysis.Statistical research was conducted using socscistatistics.com,a website with statistical calculators with accuracy verified with SPSS and Minitab statistical packages.
To compare numerical differences before and after porcelain firing, the Student's independent samples t-test was employed, with p < 0.05 considered statistically significant.Before conducting the t-test calculations, the assumption of homogeneity of variances was routinely checked through Levene's test.Welch's correction was applied to data with heterogeneous variances (W -designation used for Welsch's correction).

SEM-EDS study
This section presents the results of research on the microstructure and surface composition of elements made of both alloys in the as-cast state and after heat treatment simulating the firing procedure of ceramics, along with the differences in surface morphology and composition caused by the heat treatment.Representative images of both kinds of alloy samples in an as-cast state and after HT simulation are shown in figure 1-4, respectively.
Both control group alloys (figures 1 and 3) exhibited a solid solution matrix in a typical dendritic arrangement.Samples showed a high volume of solid solution and areas of interdendritic structures.The dendritic microstructure of the control samples is heterogeneous, as documented by the SEM-EDS studies described below.
The surface microstructure of both alloys after the ceramic firing simulation (figures 2 and 4) changes compared to the control samples.On the surface of both samples, a distinct thickened layer of oxides is visible, formed as a result of the porcelain firing processes' simulation.
In the case of Ceramic N alloy (figure 2), interdendritic areas are clearly visible, while in the case of Ivoclar 4all, the phase boundaries and the interdendritic phases themselves are less pronounced (figure 4).Few, small pores formed during the casting process are also present before and after the heat treatment.
The averaged chemical composition of the Ceramic N surface indicates the presence of a significant amount of oxygen and the absence of impurities after HT (figure 2(c)).Concentration maps of individual elements show the highest oxygen content in interdendritic precipitates (figure 2(e)).In these areas, an increase in the share of molybdenum, silicon and trace amounts of niobium is also observed.
Point EDS studies of the Ceramic N alloy microstructure after ceramic firing simulation give similar results (figure 2(d)).Point 1, located in the area of the solid solution of the γ phase, is characterized by high nickel content (55.1%) and chromium (21.6%).In the area of interdendritic precipitates (point 2) the highest concentration of molybdenum is visible −26.0%, significantly exceeding its content in analogous areas for control samples.
The results of point analysis are confirmed and supplemented by EDS analysis along the line (figure 2(f)).Along the line in the area of the interdendritic precipitate, a large increase in the content of molybdenum and a decrease in the content of Ni and Cr are visible.In general, the variation in the content of the main alloying elements after HT is more pronounced compared to the control samples.
As in the case of the Ceramic N alloy after HT, the results of the EDS analysis for the 4all alloy after HT prove the presence of oxygen and the absence of impurities (averaged EDS analysis from the area in figure 4(c)).Against the background of a solid solution rich in nickel and chromium, irregularly shaped interdendritic phases with a high content of molybdenum and silicon are visible (figure 4(e)).In the areas of interdendritic phases, the oxygen content is higher.
Point EDS analysis: The chemical composition in point 1, representative of the alloy matrix, indicates a high content of Ni and Cr, points 2 and 3 in the areas of interdendritic precipitates represent oxidized phases with a high content of molybdenum and silicon (figure 4(d)).
The linear analysis confirms the results of the above-mentioned EDS analyses (figure 4(f)).Nickel and chromium dominate in the matrix of the material, in the places of precipitates, the curves of the content of these elements drop sharply.The precipitates are mainly enriched in Mo, Si and O. Here, too, the variation in the content of alloying elements is greater than in the control sample.

Hardness test
The Vickers hardness test results for each alloy before and after the simulation of porcelain firing are presented in table 3. The hardness of Ceramic N is lower than that of Ivoclar 4all under both test conditions.According to the Student's t-test, both alloys demonstrated significantly lower values of HV hardness after the heat treatment processes.

Profilometry results
Mean Rz and Ra values and standard deviation for each group before and after the heat treatment are listed in table 4. Profilograms of all examined surfaces are shown in figure 5.
For samples of both alloys, higher values of Rz were obtained on the thermally-treated surfaces than on the as-cast surfaces.The results of testing the roughness parameters of the samples presented in table 4 do not show significant differences in the topography of the tested surfaces of the alloys.For the Ivoclar 4all alloy, a slightly higher value of the Rz parameter was noted in relation to the value of this parameter for Ceramic N.This proves that the surface of this alloy is slightly more developed.
The graphical representation of the surface profiles in the as-cast state reveals a similarity in the number of dark spots being places of depressions (figures 5(a) and (b)).After the ceramic firing simulation process, the values of the Ra and Rz parameters for both alloys are higher, with the Ivoclar 4all alloy having higher Rz and Ra

Corrosion examinations and immersion tests results
Tafel polarization curves were recorded for the tested samples of metal alloys in the potential window limited by cathode potential −150 mV OCP −1 and anode potential +150 mV OCP −1 , respectively.The obtained potentiodynamic relationships were adjusted using the AtlasLab software package to determine the corrosion current density (j corr ) and corrosion potential (E corr ) for Control and TT samples of both alloys.
Potentiodynamic polarization curves were plotted to test the wide range of potentials the samples were subjected to in a given 0,9% NaCl electrolyte.Semi-logarithmic graphs in the range from −700 mV to +1000 mV (Ag/AgCl) for both alloys in the as-cast state and after the simulation of porcelain firing are shown in figure 6.
Table 5 presents the values of corrosion current density (j corr ) and corrosion potential (E corr ) determined by means of extrapolation of the Tafel straights, respectively.E br (breakdown potential) was determined from obtained anodic polarization curves presented below in figure 6.
The corrosion currents for all alloys were of the same order of magnitude (μA/cm 2 ).However, both Ceramic N-Control and Ceramic N-TT exhibited higher values of j corr and lower E corr (table 5) compared to both Ivoclar 4all samples.For both tested alloys, the corrosion current values were higher in samples subjected to the HT process.In the case of Ceramic N_TT, the increase in the j corr value was not statistically significant, while the 4all alloy, after the porcelain firing simulation, showed a statistically significant increase in j corr .Both alloys  demonstrated significant changes in E corr values after HT, with an increase for Ceramic N, and a decrease in the corrosion potential for the Ivoclar 4all alloy.According to the corrosion current density values obtained, the corrosion resistance decreased in the following order: 4all_Control The values of the breakdown potential, indicating the loss of protective properties of the alloys' surface, decreased significantly after the simulation of porcelain firing for both tested alloys.4all_Control and after HT and Ceramic N_Control showed a passivity in a relatively wide range of potentials (table 5) however in the passive bands, the current density slightly increased linearly (figure 6).The highest E br was noted for 4all_Control, then 4all_TT and the sample of Ceramic N_Control obtained E br equal to 645 ±8.73 mV.The increase in the anode current values above the breakdown potential for these samples is an effect of the dissolution of the protective oxide layers and water oxidation (oxygen release potential in a neutral salt solution is about ∼ 0.6 V).The curves for the mentioned above samples and values of E br are not a result of localized corrosion but of uniform corrosion in the transpassive or oxygen evolution region.These results indicate that these samples are more resistant than the Ceramic N_TT sample with the lowest breakdown potential equal to (457 ±9.75 mV).
As a complementary test, electrochemical impedance spectroscopy (EIS) was used to assess the corrosion resistance of both NiCr-based alloys before and after simulating porcelain firing.Experimental results of  impedance spectroscopy, expressing the dependence of impedance on the frequency of the applied signal, are presented in Bode plots in the form of two relationships: IZI = f1 (log F) and phase angle = f2 (log F), where IZI -impedance modulus, F-frequency, (figures 7(a), (b)).For the analysis of the experimentally determined impedance spectra of the tested corrosion systems for both alloys in the initial state and after the simulation of porcelain firing, electric equivalent circuits were used, which are shown in figure 8.
Based on the analysis of the impedance spectra, it can be concluded that both alloys in their initial state exhibit higher corrosion resistance than compared to after the porcelain firing simulation process.In all analyzed spectra, only a one-time constant appears.The EIS results were satisfactorily simulated using the circuit depicted in figure 8. Its interpretation indicates that the corrosion of the tested alloys is inhibited by an oxide layer acting as a barrier.A constant phase element (CPE 1 ) was employed to fit the spectra instead of pure capacitance due to the non-ideal capacitive response.Passive films formed on both alloys before and after HT affect the resistance and pseudo-capacitance of oxide layers, where R s represents the resistance of the test electrolyte solution.The characteristics of the passive layer are reflected in the values of R 1 , which corresponds to R p (polarization resistance) and allows the assessment of the corrosion resistance of the tested samples.The fit parameters obtained are summarized in table 6.As listed in the table, according to the Student's t-test, samples of both alloys after HT demonstrated statistically lower R 1 values compared to as-cast (control) conditions.Regarding the CPE values for the alloys, though not statistically significant, there seemed to be an increase after the firing processes.
The Bode plots indicate that the impedance modulus was slightly higher for the 4all alloy than for Ceramic N in 0.9% NaCl.However, both alloys exhibited greater resistance in their as-cast states (controls) compared to the  Values Mean (SD) * Indicates statistical differences (p < 0.05) between as-cast (Control) and fired conditions (TT) (W)-Welsch's correction simulated porcelain firing condition.Prior to porcelain firing, both alloys displayed relatively high phase angles partly in the low-and mid-frequency ranges.For the control samples, the maximum phase angles were 81.1°for 4all_Control and 79.5°for Ceramic N_Control, respectively.After the HT processes, the maximum phase angles moved slightly to higher frequencies and were limited to narrower ranges of medium frequencies compared to their initial states with values of 79.3°for 4all_TT and 78.4°for Ceramic N_TT, respectively.All the spectra show that in the higher frequency region, lg Z mod tends to stay constant.In the range of average frequencies, a linear relationship between lg Z mod and frequency is observed for all samples with nearly identical values of slopes.
The results of the ions release analysis are presented in table 7.   Table 7 summarizes the results of the Ni, Mo, and Cr ions released from samples of the tested alloys before and after firing simulation.The amount of all ions released from 4all alloy was smaller when compared to the release from Ceramic N. In each of the tested cases, the concentrations of Ni, Mo and Cr ions released from the control group of both alloys were lower than from the alloys after heat treatment.For the 4all alloy, the differences are statistically significant in the case of Ni and Cr ions, and for the Ceramic N alloy, in the case of Mo ions release.It was observed that the release of Ni ions compared to other ions was greater than that of Mo and Cr.

Discussion
The paper investigated the effect of heat treatment simulating porcelain firing on selected properties of two cast Ni-Cr alloys.The results of the research carried out in this work showed that this treatment caused changes, of varying intensity, in the analyzed properties of both alloys, related to changes in their microstructure.There was a decrease in alloy hardness, changes in surface profiles represented by higher Rz and Ra values and deterioration of corrosion resistance.

SEM-EDS study
The surface microstructure of both cast alloys is characterized by a solid solution array in the dendritic disposition of the as-cast state (primary phase) (figures 1(a) and 3(a)) and a regularly distributed interdendritic (secondary) phase.EDS studies have shown that in the Ceramic N_Control samples the matrix is rich in Ni, Cr and Mo with precipitates with higher content of Mo, Si and Nb.The centers of the dendrites had lower levels of Si and Mo, as well as slightly lower levels of Cr and higher levels of Ni (figures 1(b)-(f)).The microstructure of the 4all_Control alloy is analogous, except for the absence of Nb in this alloy (figures 3(b)-(f)).Slight porosity in the interdendritic areas is visible in both alloys.The structures are similar to those described by several authors who analyzed alloys with compositions similar to those in the present work [31,60,61].
The simulation of porcelain firing resulted in the appearance of oxide layers with a high variety of compositions on the alloy's surface (figures 2 and 4).This phenomenon is more pronounced in the case of the Ceramic N alloy (figures 2(a)-(f)).In the samples of this alloy after HT, a significant increase in the content of Mo and Si in the interdendritic areas can be observed, compared to the results of the EDS analysis in these areas for control samples.Also, the oxygen content shows a large variation, which is confirmed by all types of EDS analyses carried out in this work.In the case of the 4all alloy, the same regularity occurs, although, based on elemental mapping analyses, the distribution of chromium and silicon should be considered more even than in the Ceramic N_TT alloy.
Figures 2(g) and 4(g) show microstructures of the surfaces of Ceramic N and 4all alloys, respectively, magnified to 800x, in both as-cast and after heat treatment simulating porcelain firing states.Heat treatment accentuated the differences in Ni, Cr, Mo, and Si compositions between dendritic and interdendritic regions.The surfaces of both alloys are covered with oxides, which in interdendritic areas are coarser and more cracked, consistent with the fact that in both cases the chromium content in these areas is significantly lower, especially in the case of the Ceramic N alloy.This reduction of Cr content in the surface may be due, in part, to the presence of Si on the surfaces of the fired alloys.It is known that Si is added to the bulk of the Ni-Cr alloy to reduce oxidation during casting.The temperatures reached during PFM firing may have caused Si in these alloys to migrate to the surface and react with O to form SiO 2 [50].Other researchers presented similar results regarding the microstructure of Ni-Cr alloys after processes simulating porcelain firing.Qiu reported greater grain boundary uniformity and less visible dendritic structure after HT in the Stellite N9 alloy, with a composition similar to Ceramic N [49].There have been reports of a reduced content of Cr and molybdenum (Mo) in surface oxides Values Mean (SD) * Indicates statistical differences (p < 0.05) between as-cast (control) and fired conditions (HT) (W)-Welsch's correction [50], the appearance of new phases in Ni-Cr alloys with molybdenum, and an increase in the concentration of Mo and Ni on the surface of these alloys tested by XPS and XRD methods [52].

Hardness and surface profile
The above-described changes in the alloys' structure after HT found their expression in the differentiation of the properties studied in the work.The simulation of porcelain firing changed both the hardness of alloys and their surface profiles.
In the case of both tested alloys, a significant decrease in hardness was observed after HT (table 3).This phenomenon can be explained by changes in the microstructure of the alloys, in particular homogenization in the dendrite area.The second probable reason may be the reduction in the level of post-casting stress caused by high temperatures.However, the last statement is only a hypothesis in this work, which can be checked by performing XRD tests.There are reports in the literature of a similar decrease in the hardness of Ni-Cr alloys after porcelain firing simulation processes [49,62,63], explained by the unification of the dendrite composition or the reduction of stresses [64], although some researchers reported an increase in hardness after HT [52] although the studies concerned nickel-chromium alloys with significantly different compositions than those examined in this work.

Corrosion properties and immersion test results
Another property change noted in this work relates to the surface profile.The increase in the Rz and Ra values indicates a greater surface development, which is consistent with the observed changes in its microstructure (table 4).Analysis of the EDS results indicates greater compositional diversity between dendrite areas and interdendritic spaces.It can be assumed that the oxides formed above the interdendritic areas also have different thicknesses, which causes greater variation in the surface profile.
The differences in the morphology and chemical composition of the alloy surfaces are obviously reflected in the corrosion properties.Changes in the value of corrosion current and corrosion potential after HT showing the resistance to general corrosion are statistically significant, except for the j corr value for 4all alloy, the regularity is maintained that for both tested alloys after HT, the corrosion current has higher values than for the as-cast samples (table 5).A much clearer difference in the corrosion properties appears in the analysis of the durability of the passive layers.The best behaviour was demonstrated by the 4all-Control samples with a breakdown potential of 813 ±7.68 mV and the worst behaviour by the Ceramic N_TT alloy with a breakdown potential of 457 ±9.75 mV.Both alloys exhibited significantly higher values of Ebr for the as-cast state.Generally, better corrosion resistance of 4all alloy, both in the as-cast and after HT state, especially in terms of the breakdown potential, can be explained by its higher Cr content.
Although this work did not investigate the influence of Mn on the corrosion behavior of Ni-Cr alloys, it is worth mentioning that there are reports on this subject in the literature.It was stated by Meyer in his research on the influence of various alloy additions on the corrosion resistance of Ni-Cr dental alloys [65,66] and confirmed by Pourbaix [67] that alloys with high molybdenum content and manganese addition exhibit a much wider passivation range and a better resistance to pitting in chloride-containing solutions than those without.The beneficial effect of the addition of Mn (next to Cr and Mo) on the corrosion resistance of Ni-Cr alloys was noted by Radev in a review of Ni-containing alloys for medical applications [68].Also, Bumgardner and Lucas, studying the influence of various alloy additions on the corrosion resistance of Ni-Cr alloys, stated that in Ni alloys containing 16%-27% of chromium, the addition of metals such as Mo, Mn, Ti or Cu additionally increased their corrosion resistance [69].Jones et al in turn, found that the addition of Mn in combination with Mo plays an important role in improving the quality of the passive layers [34].
A significant decrease in the value of the breakdown potential after HT should be associated with a change in the structure and chemical composition of the surface of both alloys.The appearance of much more pronounced differences in the content of Mo and Si in the interdendritic areas, compared to the cast alloys, as well as the higher oxygen content, testify to the formation of highly differentiated oxide compositions on the surfaces of the thermally treated alloys.The analysis of the alloy surface microstructure after HT and the higher values of surface roughness suggests that there is an increase in the thickness of the oxides and their thickness variation on the surface of the dendritic and interfacial areas.EDS analyses also showed slight changes in the chromium content with a decrease in its amount in the interdendritic areas.These phenomena explain the decrease in the quality of the passive layers after HT, manifested by a narrower passive range and a decrease in the value of the breakdown potential.
Similar conclusions were presented by Roach [50] and Qiu [49], who found an increase in the corrosion rate of Ni-Cr alloys after porcelain firing and at the same time a reduced content of Cr and Mo [50] and Cr, Mo and Ni [49] in the surface oxides.Wylie et al [51] studied the effect of HT on Ni-Cr alloys with varying Cr content and reported that selective corrosion was observed in regions of the microstructure containing lower levels of Cr and Mo.The results of the work by Tuncdemir et al [56], evaluating the corrosive behavior of Ni-Cr alloys, as well as Co-Cr after porcelain firing, showed that multiple firings also reduce the corrosion resistance of Cr-Co alloys.Similarly, in the works of Rylska et al [57,58] the porcelain firing simulation of Co-Cr alloys obtained by CST, MSM and SLM methods, caused, in each case, a decrease in corrosion resistance.It is essential that the most significant decrease in corrosion resistance was recorded for as-cast samples that presented the most tremendous variation in the composition of oxides on the surface after HT.
The results of the additional EIS corrosion tests are consistent with the results of the potentiodynamic tests discussed above.The polarization resistance Rp for the as-cast samples of the 4all alloy is higher (about 0.17e + 5 Ωcm −2 ) than the Rp of the Ceramic N alloy.Also, the results obtained for the R p value after simulated porcelain firing confirm the significantly reduced R p value for the TT samples compared to the control samples for both alloys by 0.22e + 5 Ωcm −2 for 4all and 0.32e + 5 Ωcm −2 for Ceramic N. In turn, the CPE values show an increase after HT.The equivalent electrical circuit fitted in our tests consists of the uncompensated solution resistance Rs connected in series with a parallel combination of solid phase element CPE 1 and charge transfer resistance R 1 .The fitting of such a substitute circuit for Ni-Cr alloys tested in environments similar to the physiological NaCl solution was reported by other researchers [26,[70][71][72][73].The value of R 1 , corresponding to R p , determines directly proportionally the degree of protection provided by the passive layer.
The Bode phase diagrams presented in figure 7 provide a clear picture of the properties of the oxide layers on the surfaces of the tested alloys.Before PFM firing, both Ni-Cr alloys showed phase angles close to 79-81°at medium and low frequencies.However, after PFM firing, the Bode phase plots showed differences from the control samples.4all showed phase angles close to 79.3°at medium frequencies, while Ceramic N showed phase angles close to 78.1°in a narrower range of medium frequencies than for 4all.For both alloys, there was a decrease in the values of phase angles in the low-frequency range, which is particularly noticeable for the Ceramic N alloy.The results of the Bode phase diagrams were also supported by the corrosion R p values for both alloys.A slight increase in the Cdl value is also observed, confirming the weakening of the quality of the passive layers (table 6).
Most literature reports on the effect of PFM on the corrosion properties of basic dental alloys are consistent with the EIS results from this work.In the works [52,74], lower values of phase angles for low and medium frequencies after HT were observed in Bode diagrams, indicating a decrease in the corrosion resistance of passive layers.The decrease in the Rp value after HT was confirmed by Qiu et al [72], who investigated the corrosion behavior of two dental nickel-chromium (Ni-Cr) alloys.Reiman [75] also found a deterioration of the passive properties of Co-Cr alloys after PFM heat treatment (lower values of phase angles, narrowing of the phase angle range for medium frequencies).Xin et al reached similar conclusions [55], comparing the surface characteristics and corrosion properties of selective laser melted (SLM) and as-cast cobalt-chromium dental alloys before and after porcelain-metal fusion (PFM).The cast alloy was characterized by a decrease in the R p value and phase angle after HT.The authors found that the oxide layers on the surface of the casting alloy after HT showed greater heterogeneity and variability of composition.It is worth noting that, in our work, the simulation of porcelain firing resulted in an increased diversity of the composition and thickness of oxide layers, especially in the case of the Ceramic N alloy.
Analogous EIS results in our work, regarding the reduction of the phase angle value and, consequently, also R p , confirm the conclusions of Xin's work.In turn, in [52], an increase in the capacity value after PFM processes was observed for Ni-Cr alloys with variable Cr and Mo contents, which is also in agreement with our results.
The results of ions release measurements are consistent with the results of electrochemical corrosion tests carried out in this research.The corrosion resistance of the Ceramic N alloy tested in this work turned out to be lower than that of the 4all alloy.The corrosion resistance parameters were the lowest for the Ceramic N sample after undergoing heat treatment.However, it should be noted that immersion tests were performed in the acidic artificial saliva, compliant with ISO standards because the corrosion of Ni-Cr alloys can be particularly visible in acidic environments.It should be presumed that in a low pH environment, slight degradation and repassivation of the alloy surfaces are responsible for modestly accelerating the release of Cr ions.
The results of the ion release study for samples before and after the porcelain firing simulation presented in this paper are consistent with the findings of other researchers comparing the type and number of ions released from Ni-Cr materials subjected to similar treatments [49,50,52].However, a quantitative comparison is not possible because the cited studies were conducted in different corrosive environments characterized by varying pH levels and immersion periods.Nevertheless, in principle, the results of these studies confirm the release of a greater number of metal ions, particularly nickel, after thermal processing of the alloys.
Despite the large amounts of released ions, we still have no clear legislative reference or epidemiological studies that would allow us to determine whether there is a toxicological risk to humans.Nevertheless, in the case of Ni, our approach should include references to Directives 94/27 and 2004/96/EC mentioned in the introduction.In light of the results obtained in this work, the use of these alloys in objects in contact with the skin or piercing elements would be prohibited.On the other side, Reclaru et al [20] testing the release of ions from dental NiCr alloys, found that their amount exceeded the permissible standards of the directives, however, they also reported that biological tests showed no cytotoxic effects on Hela and L929 cells or changes in TNF-alpha expression in monocytic cells.There was no pro-inflammatory response in endothelial cells.Therefore, no direct relationship has been demonstrated between in vitro biological evaluation tests and the corrosion characteristics of these alloys.
Released metal ions, particularly Ni ions, play a crucial role in the biological behavior of dental alloys.In this study, corrosion was assessed based on the results of electrochemical methods and the quantity of released ions.The latter method is very significant for any biological effects that corrosion may have.To evaluate the biocompatibility of the examined alloys, it is essential to determine whether they differ in terms of the amount of released Ni from other Ni-Cr-Mo alloys used for PFM.Conducted literature review allows for a comparison of the results obtained in this study with data presented by other researchers.
Since the tested alloys differed in composition, particularly in the amounts of Ni and chromium, the literature review results serving as comparative data were divided into two groups: for alloys with compositions close to the Ceramic N alloy and for alloys with compositions similar to 4all.Comparisons were made using results from studies conducted in environments with the same composition and pH, as well as under the same conditions and immersion periods as the alloys tested in this study.The results of studies on the amount of Ni ions released available in the literature show significant discrepancies, and authors did not always report the standard deviation in their works.Therefore, based on this information, it is not possible to perform a statistical analysis to determine with a certain level of confidence whether the results of this study differ significantly from published data.Nonetheless, it can be stated that the alloys examined in this work, both in the as-cast state and after heat treatment simulating porcelain firing, present amounts of released Ni ions consistent with results for other alloys, falling within the average range of Ni ion quantities.To rigorously assess whether the examined Ni-Cr alloys differ from other PFM alloys in terms of nickel dissolution, will require examining ion release for a group of selected commonly used PFM alloys and conducting statistical tests to compare the nickel dissolution rates between the examined Ni-Cr alloys and the control PFM alloys.
The possibilities of becoming sensitized to metals are diverse.Taking into account all the quantities of oral metal ingestion (e.g., the daily food consumption contains 300 to 600 μg nickel [81] the quantities of nickel dissolution from dental nickel alloys have to be discussed in comparison.In our study, the relatively corrosionresistant alloys showed a maximal loss of nickel of about 1.756 μg/cm 2 /a week, in the electrolyte used (0.1 M lactic acid/sodium chloride).With regard to the nickel content in food and drinking water, these alloys can be considered to be not critical.In principle, it seems that Ni-Cr alloys in the oral environment only cause soft tissue inflammation and sensitization dermatitis if patients have already been sensitized before or if the substance loss of the used alloy is very high.
It is very important to increase the awareness of prosthetic patients about the potential dangers of using nickel-containing alloys.The measures implemented by the ADA, which require that nickel-containing alloys be labelled with a warning indicating that they should be avoided in patients with a confirmed nickel allergy, should be universally adopted [82].
There are also opinions that nickel compounds are generally very well soluble in water and the risk of allergy in the oral cavity is extremely low.Setcos et al [83] based on a review of the literature on the biological reaction to dental alloys containing Ni, concluded that dental alloys containing Ni do not pose a risk to patients.Ch.Achitei and others in their work [84] report that despite the relatively frequent occurrence of skin contact allergies to Ni, it has been clinically observed that the use of Ni-Cr alloys in the oral cavity does not systematically cause allergic reactions.Therefore, they conclude that any potential allergy occurs only in very sensitive individuals.Considering the small amounts released and the short biological half-life of Ni, a systemic toxic attack should be excluded.However, local toxic effects cannot be ruled out, which may also occur in the case of other metals.

Limitations of the study
This work has some limitations.The tests evaluated only the behavior of the two alloys produced by casting.A wider range of materials and especially manufacturing methods should therefore be explored.It should be mentioned here that the methods of producing elements from Ni-Cr and Co-Cr alloys, such as soft milling (SM) and selective laser melting (SLM), ensure obtaining structures of high homogeneity [85], also Yun et al evaluated the in vitro biocompatibility of Ni-Cr alloys produced by casting, SLM and SM and reported that the EDS maps of the SLM and SM alloys exhibit a more homogeneous elemental dispersion than the cast alloy's EDS map.They also stated that the amount of Ni and Mo ions released from the SLM and SM samples was lower than that from the cast alloy [31] despite a significantly higher level of porosity observed in SLM elements, which was presented by the same authors in [86].Considering these results, it would be advisable to investigate whether the heat treatment accompanying porcelain firing will contribute to increasing the heterogeneity of oxide layers on the surface and to reducing the corrosion resistance of Ni-Cr alloys investigated by us and produced by these methods.Additionally, in the work by Ming et al it was shown that the pretreatment of dental Ni-Cr-Mo alloys reduced the number of released ions and increased their corrosion resistance [87].It seems appropriate to perform an analogous pretreatment for the alloys investigated in this study before simulating porcelain firing to see if it improves their corrosion resistance and reduces the amount of released metal ions, particularly Ni after heat treatment.
In addition, further research is needed to investigate the oxide layer structures and composition in detail (i.e. XPS).Moreover, it is also necessary to conduct surface studies of the alloys (microstructural and chemical composition) in the as-cast state and after porcelain firing simulation following the immersion test in selected electrolyte solutions with varying pH levels, appropriate to the environment found in the oral cavity.Such studies will allow for a precise determination of the type of corrosion and the areas where it occurred.

Conclusions
Based on the results obtained in this study, a general conclusion can be drawn regarding the hardness, profile changes and corrosion properties of the two commercial Ni-Cr alloys before and after repeated porcelain firing cycles.
-After heat treatment simulating porcelain firing, both alloys showed significantly reduced hardness.
-Heat treatment processes decreased the overall resistance to general corrosion, significantly worsened the resistance of surface passive layers and caused the increased number of ions released from the alloys, however, the amounts of released Ni ions for all tested alloy variants do not show a significant difference compared to the amounts of Ni ions released by other Ni-Cr alloys used for the PFM.
-After HT processes, differentiation of surface profiles and an increase in their roughness as well as a significant increase in heterogeneity of oxide layers on the surface were found.
-These two phenomena may be responsible for the significant decrease in the resistance of the alloys to electrochemical corrosion, and in particular for the deterioration of the properties of the passive layers.
-No impurities resulting from porcelain firing were observed on the surface of Ni-Cr alloys.

Figure 1 .
Figure 1.Ceramic N_Control sample: (a) SEM-SE image of sample's surface (b) SEM-SE micrograph showing the morphology with selected objects for EDS analysis (c) average EDS results from the marked area in figures 1(b), (d) EDS results for marked points in figures 1(b); (e) maps presenting the surface distribution of Ni, Cr, Mo, Si, Nb and Fe; (f) SEM-SE micrograph with corresponding EDS line scan results for the marked line in figure 1(b).

Figure 2 .
Figure 2. Ceramic N_TT: (a) SEM-SE image of sample's surface (b) SEM-SE micrograph showing the morphology with selected objects for EDS analysis (c) average EDS results from the marked area in figures 2(b), (d) EDS results for marked points in figures 2(b); (e) maps presenting the surface distribution of Ni, Cr, Mo, Si, Nb and O; (f) SEM-SE micrograph with corresponding EDS line scan results for the marked line in figures 2(b); (g) comparison of alloy structures before (control) and after HT with the given composition in selected micro-areas.

Figure 3 .
Figure 3. 4all_Control sample: (a) SEM-SE image of sample's surface (b) SEM-SE micrograph showing the morphology with selected objects for EDS analysis (c) average EDS results from the marked area in figures 3(b), (d) EDS results for marked points in figures 3(b); (e) maps presenting the surface distribution of Ni, Cr, Mo and Si; (f) SEM-SE micrograph with corresponding EDS line scan results for the marked line in figure 3(b).

Figure 4 .
Figure 4. 4all_TT: (a) SEM-SE image of sample's surface (b) SEM-SE micrograph showing the morphology with selected objects for EDS analysis (c) average EDS results from the marked area in figures 4(b), (d) EDS results for marked points in figures 4(b); (e) maps presenting the surface distribution of Ni, Cr, Mo, Si and O; (f) SEM-SE micrograph with corresponding EDS line scan results for the marked line in figures 4(b); (g) comparison of alloy structures before (control) and after HT with the given composition in selected micro-areas.

Figure 5 .
Figure 5. Profilometric analysis of the 600 × 650 mm area coincident with the center of the samples investigated.

Figure 6 .
Figure 6.Potentiodynamic polarization curves of NiCr-based alloys (Control and TT) tested in 0,9% NaCl, on semi-logarithmic axes in relation to the Ag/AgCl reference electrode.

Figure 7 .
Figure 7. Bode plots of impedance spectra (phase angle and impedance modulus) of (a) Ceramic N alloy corrosion systems before and after HT and (b) 4all alloy corrosion systems before and after HT.

Figure 8 .
Figure 8. Electrical equivalent circuits of examined corrosion systems.

Table 2 .
Firing simulation parameters for porcelain.

Table 3 .
Hardness test results for the examined alloys.

Table 4 .
Rz and Ra results.

Table 5 .
Values of j corr , E corr and E br from the electrochemical examination.

Table 6 .
Electrical parameters of equivalent circuit elements of the tested corrosion systems.